Process for producing a cross-linked copolymer of tetrafluoroethylene and propylene using high energy ionizing radiation

ABSTRACT

A cross-linked copolymer of tetrafluoroethylene and propylene is produced by irradiating a copolymer of tetrafluoroethylene and propylene in the absence of monomers of tetrafluoroethylene, propylene, or other copolymerizable monomer, with high energy ionizing radiation.

Umted States Patent 1 1 [111 3,718,558 Tabata et al. 1451 Feb. 27, 1973 PROCESS FOR PRODUCING A CROSS- [56] References Cited LINKED COPOLYMER OF TETRAFLUOROETHYLENE AND UNTED STATES PATENTS PROPYLENE USING HIGH ENERGY 3,556,965 1 1971 DAgostino et al ..204 159.2 IONIZING RADIATION 3,467,635 9/1969 Brasen et al ..260/80.76

[75] Inventors: Yoneho Tabata, Matsudo; Gen OTHER PUBLICATIONS Kojima, Tokyo, both of Japan Tabata et al., Radiation Induced Copolymorization of [73] Assisnee; Asahi Glass Company Ltd" Tokyo, TFE With Propylene at Low Temperature, J. of Japan Polymer Science; part 4, Vol. 2, pp. 2235-2243 (1964) [22] Filed: Dec. 11, 1970 [21] Appl No; 97,387 Primary Examiner-Murray Tillman Assistant Examiner-Richard B. Turer Attorney-Oblon, Fisher & Spivak [30] Foreign Application Priority Data Dec. 11, 1969 Japan ..44/99615 [57] ABSTRACT A cross-linked copolymer of tetrafluoroethylene and [52] US. Cl. ..204/159.2, 260/875 B propylene is produced by irradiating a copolymer of tetrafluoroethylene and propylene in the absence of [51] Int. Cl.. ..C08d 1/00, C08f l/16 monomers of tetrafluoroethylene, propylene, or other V V copolymerizable monomer, with high energy ionizing [58] Field of Search ..204/159.22, 159.2; 260/928 radiation 6 Claims, No Drawings PROCESS FOR PRODUCING A CROSS-LINKED COPOLYMER OF 'IETRAFLUOROETIIYLENE AND PROPYLENE USING HIGH ENERGY IONIZING RADIATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a technique for cross-linking copolymers of tetrafluoroethylene and more particularly to a technique using high energy ionizing radiation.

2. Description of the Prior Art Copolymers of tetrafluoroethylene and propylene, known as fluorine-containing elastomers, are disclosed in U.S. Pat. No. 3,467,635. In that reference, it is disclosed that copolymers of tetrafluoroethylene and propylene can be cross-linked with a curing agent such as a peroxide or an amine. When these types of curing agents are used, however, the curing agent becomes incorporated into the resultant cross-linked copolymer, so that its characteristic heat resistance, mechanical strength and/or electrical properties are adversely affected. Moreover, it has been found that conventional cross-linking agents provide only reduced cross-linking efficiency when used for cross-linking copolymers of tetrafluoroethylene and an olefin and the degree of cross-linking with conventional cross-linking agents is usually insufficient to provide a copolymer of adequate mechanical strength.

In order to provide a copolymer of tetrafluoroethylene and propylene of high mechanical strength, it has been heretofore necessary to provide a terpolymer with a suitable cure-site monomer. Upon cross-linking the terpolymer, the cure-site monomer provided sufficient cross-linking to yield a high mechanical strength fluorine containing elastomer. A

need exists, however, for a copolymer elastomer having good mechanical and electrical properties.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a novel cross-linked copolymer of tetrafluoroethylene and propylene having high heat resistance, good mechanical strength and excellent electrical properties.

It is a further object of the present invention to provide a cross-linked copolymer of tetrafluoroethylene and propylene which contains no impurity which might reduce the excellent properties of the copolymer.

It is a still further object of the present invention to provide a process for producing a cross-linked copolymer of tetrafluoroethylene and propylene with high cross-linking efficiency.

A further object of this invention is to provide a cross-linking process which can be used for shaped articles composed of copolymers of tetrafluoroethylene and propylene.

These and other objects have now herein been attained by irradiating the copolymer of tetrafluoroethylene and propylene with high energy ionizing radiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The tetrafluoroethylene-propylene copolymer used in the present invention can be provided by conventional bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, or the like. Also, these copolymers can be produced by using conventional catalyst systems, using a polymerization initiator, thermopolymerization, photopolymerization, or polymerization by use of ionizing radiation, etc. Copolymers of tetrafluoroethylene and propylene which contain minor amounts of other copolymerizable components, such as ethylene, isobutylene, acrylic acid, vinylfluoride, hexafluoropropylene, vinylidene-fluoride, chlorotrifluoroethylene, and chloroethylvinylether, etc., can also be used in the present invention.

The molar ratio of tetrafluoroethylene to propylene in the copolymer can be selected over broad ranges, such as tetrafluoroethylene/propylene (molar ratio) or 90/10 20/80, and preferably /30 30/70, especially 60/40 45/55. The molecular weight of the copolymer and the molar ratio of the components can be selected according to the properties and applications required of the end product.

In the process of the present invention, it is preferable to use a copolymer of tetrafluoroethylene and propylene having a M.W. of greater than 20,000 and having an intrinsic viscosity in tetrahydrofuran at 30 C. of greater than 0.25.

It is rather difficult to cross-link this type of copolymer if the M.W. is less than 20,000 to form a high mechanical strength copolymer even with the use of ionizing radiation. For example, a copolymer of tetrafluoroethylene and propylene which has an M.W. of about 18,000, cross-linked with 10 50 Mrad of high energy ionizing radiation is characterized by an ultimate tensile strength of only 38 45 kglcm It is possible to yield a cross-linked copolymer havin an ultimate tensile strength of greater than 50 kg/cm, however, and especially more than kg/cm without any reinforcing material, by using copolymers of tetrafluoroethylene and propylene having molecular weights of greater than 20,000 when treated in accordance with the process of the present invention. Suitable high energy ionizing radiation for the present invention includes a-rays, B-rays, a-rays, X-rays, accelerating particle rays, neutron rays, and electron beams. It is preferable, however, to use a higher energy form of ionizing radiation. Any type of ionizing radiation source can be used, e.g., cobalt-60, cesium-137 and krypton-83, atomic energy generator, such as an atomic reactor, various types of particle accelerators, X-rays generators, and electron beam generators, etc. In general, it is preferable to use a-rays from a cobalt-60 source, accelerating particle rays and electron beams.

The dose rate and total dose of the high energy ionizing radiation are not particularly limited; however, when the total dose is too low, it is difficult to provide sufficient cross-linking and the desirable ultimate tensile strength can not be obtained. The dose rate is selected depending upon the time of radiation and other factors. When the dose rate is too high, the physical properties of the resultant cross-linked copolymer will be adversely affected, while when the dose rate is in the absence of any monomers of tetrafluoroethylene, too low, cross-linking occurs too slowly. -propylene, or other copolymerizable monomer, etc., In the industrial operation of this process, it is es ciall when the copolymer is in the form of a preferable to use a dose rate in the range of 10 10 shaped article. roentgens/hour, and preferably 10 5 X roent- 5 The copolymer can be subjected to the radiation gens/hour. The total dose should be in the range of treatment in air; however, it is preferable to effect about 10 l0 rads. In general, the particular dose and radiation in vacuum or in argon, helium or nitrogen. It dose rate will depend upon the particular molecular is also possible to irradiate the copolymer in water. The weight of copolymer. cross-linking reaction is usually conducted in high effi- It has been found that an excess of irradiation will 10 ciency at room temperature. However, it is possible to cause a decrease in the ultimate elongation of the irradiate at about 100C.or more. 1

resultant cross-linked copolymer and a decrease in the copolymers of tetrafluoroethylene and p py ultimate tensile strength and the heat resistance of the Whhih are difficuh t0 Cross-link y e tional copolymer. l5 methods, can be readily cross-linked using the It is not clear why the ultimate t n il t th d techniques of the present invention to yield a product the decomposition t m atur f h resulting crosshaving an ultimate tensile strength of greater than 50 linked copolymer are decreased by an excess amount g/ an ultimate elongation of greater than 200 P of ionizing radiation. However, it is presumed that main cent, and a decomposition temperature of more than chain scission of the copolymer occurs concurrently and which is further characterized by good with the cross-linking reaction. For example, when a heat reslstahcelow molecular weight copolymer of tetrafluoroethylene Having generally descrihed the invention a f" and propylene is irradiated, the ultimate tensile understanding can be obtained by reference to certain strength of the resulting cross-linked copolymer will Speclfic E whlch are Prollded herein for P usually be too low. If excess cross-linking occurs, such R R lnusmmon only and are not mended to be as when it is desired to increase the ultimate tensile hmmngm any manner' strength, the ultimate elongation will become too low.

It is difficult to increase the ultimate tensile strength EXAMPLES l 8 of the resulting copolymer beyond a specific degree by Hi h energy ionizing radiation from a cobalt-60 increasing the total dose of ionizing radiation, Since source or an electron beam was used to irradiate each main chain scission will occur which will adversely affil f a copolymer of tetrafluoroethylene and fect the ultimate tensile strength. This might be the len average molecular weight of about 30,000, reason why a high ultimate tensile strength cross-link d intrinsic viscosity in tetrahydrofuran at 30 C. of 0.36 copolymer cann be prod ed ing lin r (molar ratio of C,F.,:C,ii of 51:49) which resulted in copolymers of tetrafluoroethylene and propylene of the formation of the following cross-linked copolymer. less than 20,000 molecular weight The properties of each cross-linked copolymer were Excess irradiation will also cause main chain scission measured t id th foll wing results as shown in of copolymers having molecular weight of more than T bl 1. 20,000, thereby causing undesirable decreases in the As the test of physical properties, ultimate tensile mechanical properties of the resultant copolymer, and strength (kg/cm) and ultimate elongation (percent) accordingly, it is important that the total dose be careere measured at 500 mm./min. velocity at 25 C. fully controlled. As specified above, in general the total Thermal decomposition temperature C.) w

dose required is dependent upon the molecular weight sured by use of a thermo-balance.

of the copolymer. It is preferable to use a dose of 10 -5 It was found that this copolymer had an ultimate ten- X 10 rads, and especially 5 40 Mrads to produce a sile strength of 3 kg/cm, an ultimate elongation of 300 cross-linked copolymer of tetrafluoroethylene and percent, and a decomposition temperature of 360 C.

TABLE "1 Condition of irradiation Cross-linked copolymer Irradi- Decomation Total Elonposition Type of radiotemp. Energy dose Strength gation temp. No. active rays C.) Atmosphere Dose rate (m.e.v.) (MratL) (kg/cm!) (percent) C.)

1 'y-rays 25 low/hr" 1.25 20 700 345 2 (lo 25 l. '25 40 80 210 350 3 d0 25 l. 25 2O 800 340 4 do do do 1.25 5 100 500 330 5 Electron l)en1n 2540 In nir 0.5 #A./cm. 1.5 20 )5 850 340 25-40 do 1.5 IL/em 1.5 90 10 650 335 25-40 ..1. do 0.5 uA./cm. 2. 5 25 90 700 340 2540 do 0.5 nA./cm. l. 5 40 85 250 330 propylene having an ultimate tensile strength of more EXAMPLES 9 13 than 50 kg/cm and preferably more than 80 kg/cm, an ultimate elongation of more than 200 percent, and a Various copolymers of tetrafluoroethylene and decomposition temperature of more than 300 C. propylene having various average molecular weights he pr cess of the present en n ca be applied 65 were respectively shaped into sheets having 1 mm. of to various forms of tetrafluoroethylene-propylene thickness. a-Rays from a cobalt-60 source were used to copolymers suchasshaped articles of film,sheet, pipe, irradiate the samples at a dose rate of 10 roentrods, rings, coated membranes, powders or granules. It gens/hour in vacuum at a total dose of 10 Mrad. to is preferable, however, to apply the radiation treatment result in a cross-linked copolymer. Each cross-linked TABLE2 Copolymer Cross-linked Copolymer No.

' Average intr- Strength Elonga- Decommolecular insic (kg/cm) tion position weight visco- C,FJ ($1) temperasity C,l-l, ture ("C.) 9 1.8 xl0.22 50/50 38 480 350 10 2.0 xl00.25 51/49 50 480 350 11 3.0 x100.36 51/49 163 213 345 12 5.0 x100.55 52/48 130 500 350 13 6.7 x100.72 51/49 137 495 350 EXAMPLES l4 18 A copolymer of tetrafluoroethylene and propylene having an average molecular weight of 3,000, an intrinsic viscosity in tetrahydrofuran at 30 C. of 0.36, and a molar ratio of C,F.,:C H of 51:49 was formed into a sheet as shown in Example 9. a-Rays from a cobalt-60 source, dose rate of 10' roentgens/hour, were used to irradiate each sample in air at room temperature to result in cross-linkage.

The physical properties of resultant cross-linked copolymers obtained under various total doses were measured by the test methods shown in Example 1 to provide the following results as shown in Table 3.

TABLE 3 Cross-linked Copolymer No. Total strength Elonga- Decomposition Dose tion temperature (Mrad.) (kg/cm) (C.) 14 55 1,000 355 l5 l0 80 480 355 16 20 101 313 345 17 40 l 17 247 340 18 50 100 205 3 30 EXAMPLES l9 23 Electron beams of 1.5 MeV energy and 0.48 nA/cm electron density were used to irradiate sheets of copolymers of tetrafluoroethylene and propylene shown in Example 14 in air at room temperature to result in cross-linkage.

The physical properties of resultant cross-linked copolymers obtained at various doses were measured by the test methods shown in Example 1 to provide the following results shown in Table 4.

TABLE4 CrossIinked Copolymcr No. Total Dose strength Elonga- Decomposition (Mrad) (kg/cm) tion (36) temperature l9 5 52 1200 360 20 1O 65 530 3 52 21 20 373 344 22 40 105 270 346 23 50 76 220 340 Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention.

What is claimed as new and intended to be covered by letters patent of the United States is:

1. A process for producing a cross-linked copolymer of tetrafluoroethylene and propylene which comprises subjecting a copolymer of tetrafluoroethylene and propylene, wherein the mole ratio of the tetrafluoroethylene to propylene is from /10 to 20/80, the average molecular weight is greater than 20,000, and the intrinsic viscosity in tetrahydrofuran at 30 C is more than 0.25, to high energy ionizing radiation at a dose rate of from 10 l0 roentgens/hour and a total dose of l0 l0 rads.

2. The process of claim 1 in which said copolymer is irradiated in the absence of any monomer of tetrafluoroethylene, propylene, or other copolymerizable monomer.

3. The process of claim 1, in which a shaped copolymer of tetrafluoroethylene and propylene is exposed to the irradiation treatment.

4. The process of claim 1, wherein the total dose is from 10 5 X 10' Rads.

5. The process of claim 1, wherein said copolymer is irradiated in vacuum.

6. A cross-linked copolymer of tetrafluoroethylene and propylene which is characterized by an ultimate elongation of greater than 200%, an ultimate tensile strength of greater than 80 kg/cm and a decomposition temperature of more than 300 C. which is produced by irradiating a copolymer of tetrafluoroethylene and propylene having an average molecular weight of more than 20,000 and an intrinsic viscosity in tetrahydrofuran at 30 C. of more than 0.25, wherein the mole ratio of tetrafluoroethylene to propylene is from 90/10 to 20/80, with high energy ionizing radiation at a total dose of 10 5 X 10" Rads, and a dose rate of 10 to 10 roentgens/hour. 

2. The process of claim 1 in which said copolymer is irradiated in the absence of any monomer of tetrafluoroethylene, propylene, or other copolymerizable monomer.
 3. The process of claim 1, in which a shaped copolymer of tetrafluoroethylene and propylene is exposed to the irradiation treatment.
 4. The process of claim 1, wherein the total dose is from 105 -5 X 107 Rads.
 5. The process of claim 1, wherein said copolymer is irradiated in vacuum.
 6. A cross-linked copolymer of tetrafluoroethylene and propylene which is characterized by an ultimate elongation of greater than 200%, an ultimate tensile strength of greater than 80 kg/cm2, and a decomposition temperature of more than 300* C. which is produced by irradiating a copolymer of tetrafluoroethylene and propylene having an average molecular weight of more than 20,000 and an intrinsic viscosity in tetrahydrofuran at 30* C. of more than 0.25, wherein the mole ratio of tetrafluoroethylene to propylene is from 90/10 to 20/80, with high energy ionizing radiation at a total dose of 105 - 5 X 107 Rads, and a dose rate of 102 to 109 roentgens/hour. 